The present invention relates to a prism plate which efficiently emits within a predetermined range of angles a flux of light that is incident over a wide range of angles, to an indicator, such as a liquid crystal indicator, incorporating such a prism plate that is used for information indication in information processors such as personal computers, word processors, and the like, and to an indicator illumination method which efficiently emits the flux of light within the predetermined range.
It has been desired to develop portable information processors such as personal computers and word processors, with the growth in an information-oriented society. Performance required for the portable information processors is that they should be small in size, light in weight, and consume a small amount of electric power so that they can be used for a long time on a small capacity power source.
In portable information processors, in particular, the information indicator unit roughly determines the shape of the apparatus and at ways consumes electric power; hence it is desired that such unit have a small thickness and be of a type which consumes only a small amount of electric power.
Such an indicator can be represented by a liquid crystal indicator of back-light type as described in Japanese Patent Laid-Open No. 67016/1992.
Referring to FIGS. 1 and 2 of Japanese Patent Laid-Open No. 67016/1992, the light rays emitted from light sources 23 and 24 and the light rays reflected by reflectors 25 are scattered by a scattering plate 26, pass through optical device 27 such as a prism plate whose light incident side is a flat plane 28 and whose light outgoing side is a prism plane 29, and are incident on a liquid crystal indicator element 12. In an embodiment of Japanese Patent Laid-Open No. 67016/1992, the optical device 27 such as the prism plate is made of a polycarbonate resin or a like resin, and the vertex angle between two planes of each prism on the prism plane 29 is, for example, 90 degrees (a half vertex angle is 45 degrees). The optical device 27 such as the prism plate disposed on the scattering plate 26 can collect light scattered over a wide range of angles by the scattering plate 26 in the normal direction of the optical device 27, increasing the luminance within an effective range of visual angle.
A problem arises, however, with a conventional prism plate made of a polycarbonate resin or a like resin having a vertex angle of about 90 degrees (with a half vertex angle of 45 degrees) as shown in the above-mentioned Japanese patent document.
FIG. 6 is a sectional view illustrating the conventional prism plate on an enlarged scale. Reference numeral 60 denotes a prism plate and 61 denotes a ray of light. A solid line 70 in FIG. 7 represents the visual angle characteristic of outgoing light of the prism plate 60 illustrated in FIG. 6, which characteristic is found by a ray tracing calculation under the condition where the vertex angle 2.theta. between a prism plane 62 and a prism plane 63 is 90 degrees (a half vertex angle .theta.=45 degrees) and the refractive index n is 1.585. A dotted line 71 represents the angle dependency of light intensity of the ray of light 61 incident on the prism plate 60 which corresponds to the visual angle characteristic as if there is no prism plate 60. As shown by the solid line 70 in FIG. 7, the luminance is increased by more than 1.4 times over an effective range of visual angle of from -35 degree to 35 degrees. However, the light is also emitted over ineffective ranges of visual angle from -80 degrees to -60 degrees and from 60 degrees to 80 degrees resulting in wasteful consumption of electric power. When viewed from these directions, the indicator is unnecessarily bright. The cause is attributed, as shown in FIG. 6, to the fact that the ray of light 61 incident on the prism plate 60 is totally reflected by the prism plane 62 and is emitted from the prism plane 63.
An object of the present invention is to solve the above-mentioned problems by providing an indicator which prevents light from being emitted into ineffective ranges of visual angle, so as to increase the intensity of light emitted within an effective range of visual angle. It is another object of the present invention to provide an indicator which has a small thickness and consumes a small amount of electric power, thereby being particularly suitable for portable information processors.
In order to achieve the above objects, the basic concept of the present invention is the recognition that the prism plate gathers the light from the scattering plate in the direction of an effective visual angle (for example from -60 degree to +60 degree as shown in FIG. 7), and minimizes intensity of outgoing light in the direction of a noneffective visual angle. And more desirably, the present invention is effective to assure that there is no peak of the intensity of outgoing light in the direction of the noneffective angle.
Another aspect of the present invention is a prism plate having prism-like protuberances formed at a side thereof, in which the vertex angle 2.theta. formed by two planes of each prism-like protuberance is defined by the formula, EQU 2.theta.&gt;2/3(.pi./2+2.times.arcsin(1/n))
where n is the refractive index of the prism plate medium.
According to yet another feature of the present invention, an data indicator for an information processor comprises a light source, a scattering plate which scatters light from the light source, an indicator plate irradiated with scattered light from the scattering plate, and a prism plate having many prism-like protuberances formed on one side thereof and being disposed between the scattering plate and the indicator plate such that the prism-like protuberances face the scattering plate, and, the vertical angle 2.theta. formed by two planes of each of the prism-like protuberances is, EQU 2.theta.&gt;2/3.times.(.pi./2+2.times.arcsin (n'/n))
where n is the refractive index of the prism plate, and n' is the refractive index of the material in which the prism plate is arranged.
Accordingly, in a prism plate whose prism vertex angle 2.theta. satisfies the above inequalities, the totally reflected scattered light from one surface of the prism-like protuberance is also totally reflected on the other surface of the prism-like protuberance, thereby making it possible to gather the emitted light only in the effective range of visual angle.